What happens when you create a Linux user?

Source code: https://gist.github.com/sebastiancarlos/1a5b04099b750f5a15c77c8cb4f03fc8

### Sample output:
#
# CREATE_HOME        no
# CREATE_MAIL_SPOOL  no
# LOG_INIT           yes
# USERGROUPS_ENAB    yes
# HOME               /home
# MAIL_DIR           /var/spool/mail
# MAIL_FILE          N/A
# SHELL              /bin/bash
# SKEL               /etc/skel
# EXPIRE             N/A
# GROUP              984 (users)
# INACTIVE           -1
# PASS_MAX_DAYS      99999
# PASS_MIN_DAYS      0
# PASS_WARN_AGE      7
# UMASK              077
# -------------------------------------------------------------------------------------
# CREATE_HOME:       Create a home directory if it does not exist.
# CREATE_MAIL_SPOOL: Create a mailbox file for the user.
# LOG_INIT:          Add the user to the lastlog and faillog databases.
# USERGROUPS_ENAB:   If set to yes, useradd will create a group with the name of the
#                    user, and userdel will remove the user's group if it contains no
#                    more members.
# HOME:              The user's name will be affixed to form the new home.
# MAIL_DIR:          The mail spool file will be created using this value.
# MAIL_FILE:         Same, but relative to the user's home directory.
# SHELL:             The user's login shell.
# SKEL:              The skeleton directory, which contains files and directories to
#                    be copied into the user's home directory.
# EXPIRE:            The date on which the user account will be disabled, if any.
# GROUP:             If USERGROUPS_ENAB is set to no, or if the flag -N/--no-user-group
#                    is passed, this will be the new user's group.
# INACTIVE:          The number of days after a password has expired in which the
#                    password is still be accepted. -1 disables the inactive period.
# PASS_MAX_DAYS:     The number of days a password may be used. If is older, a password
#                    change will be forced.
# PASS_MIN_DAYS:     The minimum number of days allowed between password changes. Any
#                    attempt sooner will be rejected.
# PASS_WARN_AGE:     Warn user N days befere password expiration.
# UMASK:             The file mode creation mask is initialized to this value.